1. Field of the Invention
The present invention relates to a new process for the preparation of polymers containing hydroxyl groups by the alkoxylation of copolymers of acrylic acid and/or methacrylic acid with other olefinic compounds using special sulphur-containing catalysts, and the use of these polymers as reaction partners for organic polyisocyanates in the production of polyurethane plastics, in particular lacquers and coatings.
2. Description of the Prior Art
Polymer resins containing hydroxyl groups based on olefinically unsaturated monomers and their use as binders for coating materials have been described in numerous publications (see e.g. DE-OS Nos. 2,460,329, 1,038,754, U.S. Pat. Nos. 3,002,959, 3,375,227, GB-PS No. 1,515,868).
These known polymer resins containing hydroxyl groups are generally prepared by the use of hydroxyalkyl esters of acrylic acid and/or methacrylic acid as comonomers. This method is complicated since these special comonomers containing hydroxyl groups must first be prepared by a costly preliminary process.
The present invention was directed to providing a new, simple process for the preparation of polymer resins containing hydroxyl groups which could be used in the same manner as the known hydroxyl-containing polymer resins of the current state of the art in combination with organic polyisocyanates for producing high quality polyurethane resins, in particular coatings.
This problem could be solved by the process according to the invention described below wherein polymer resins containing carboxyl groups are converted into the corresponding polymer resins containing hydroxyl groups by the alkoxylation reaction. The advantage of the process according to the invention compared with the state of the art is that preparation of the polymer resins does not require the use of hydroxyalkyl esters of acrylic or methacrylic acid previously prepared by an expensive procedure in a separate reaction step. Instead, the starting materials used according to the invention are copolymers of the above mentioned acids with other olefinically unsaturated monomers.
It has previously been known that compounds containing carboxyl groups could be converted into the corresponding compounds containing hydroxyalkyl groups in an aqueous or organic medium or solvent free with alkylene oxides such as ethylene oxide, propylene oxide or 1,2-butylene oxide in the presence of basic catalysts such as tertiary amines, tetraalkylammonium hydroxides, alkali metal or alkaline earth metal oxides or hydroxides, basic ion exchangers or simple thioethers such as thiodiglycol (see e.g. U.S. Pat. No. 3,116,270, BE-P No. 634,310, BE-P No. 590,775, GB-P Nos. 771,569, 940,766, 1,002,343, DE-OS No. 3,148,022 or DE-AS No. 1,248,660). However, the basic substances accelerate not only the reaction of carboxyl groups with epoxides, but also ester interchange and trans-esterification reactions as well as the addition the epoxides to hydroxyalkyl groups. This means that in the alkoxylation of polymers containing carboxyl groups, the reaction of substantial proportions of carboxyl groups requires the use of substantially more than the equivalent quantity of epoxide, that the end products have an undesirably high ether group content, and that linkages are formed between the polvmer chains which may lead to undesirable increases in viscosity even to the extent of causing gelation. In addition, undesirable discolorations are liable to occur in the case of aminic catalysts. Although the use of simple thioethers such as thiodiglycol (DE-AS No. 1,248,600) as catalysts is effective for the alkoxylation of simple carboxylic acids such as formic acid, acetic acid, adipic acid or benzoic acid, to name but a few, such simple thioethers have not hitherto been recommended for the alkoxylation of copolymers containing carboxyl groups. The thioethers recommended as catalysts in DE-AS No. 1,248,660 would in any case be to a large extent unusable for this purpose since the thio compounds, which are in most cases malodorous, could not be removed from the end products such that they become unusable for the intended purpose (production of polyurethane lacquers).
The solution according to the invention to the discussed problem entails carrying out the alkoxylation of the polymers containing carboxyl groups in the presence of selected catalysts.